1. Field of the Invention
The present invention relates to a method for manufacturing circuit devices, and particularly, to a method for forming a low-profile circuit device using two conductive films laminated via a third conductive film to serve as a barrier layer in an etching step.
2. Description of the Related Art
In recent years, IC packages have increasingly been used in portable equipment and small-sized high-density mounting equipment, and conventional IC packages and mounting concepts have undergone drastic changes. This has been mentioned in, for example, Japanese Unexamined Patent Publication No. 2000-133678, which is a technique related to a semiconductor device employing a polyimide resin sheet of a flexible sheet as an example of its insulating resin sheet.
In FIG. 15 through FIG. 17, a flexible sheet 50 is employed as an interposer substrate. Herein, drawings shown in the upper part of the respective drawings are plan views, drawings shown in the lower part are sectional views along a line A—A.
First, on the flexible sheet 50 shown in FIG. 15, a copper foil pattern 51 is prepared by being adhered via an adhesive. This copper foil pattern 51 is different in its pattern depending on whether a semiconductor element to be mounted is a transistor or an IC, and in general, bonding pads 51A and an island 51B are formed. In addition, a symbol 52 shows an opening portion to lead out an electrode from the rear surface of the flexible sheet 50, and the copper foil pattern 51 is exposed therethrough.
Next, this flexible sheet 50 is transferred to a die bonder, and as shown in FIG. 16, semiconductor elements 53 are mounted. Thereafter, this flexible sheet 50 is transferred to a wire die bonder, and the bonding pads 51A and pads of the semiconductor elements 53 are electrically connected by metal wires 54.
Lastly, as in FIG. 17A, a sealing resin 55 is provided on the front surface of the flexible sheet 50 for sealing. Herein, transfer molding is performed so as to cover the bonding pads 51A, island 51B, semiconductor element 53, and metal wires 54.
Thereafter, as shown in FIG. 17B, connecting means 56 such as solder or solder balls are provided, and as a result of passing through a solder reflow furnace, spherical solder 56 fusion-bonded with the bonding pads 51A via the opening portions 52 are formed. In addition, since the semiconductor elements 53 are formed in a matrix shape on the flexible sheet 50, dicing is performed as in FIG. 16 to separate the semiconductor elements individually.
In addition, in the sectional view shown in FIG. 17C, 51A and 51D are formed as electrodes on both surfaces of the flexible sheet 50. In general, this flexible sheet 50 is supplied after patterning of both surfaces by a manufacturer.
A semiconductor device using the above-described flexible sheet 50 uses no widely-known metal frame and, therefore, has an advantage such that an extremely small-sized low-profile package can be realized, however, substantially, wiring is carried out by only one-layer copper pattern 51 provided on the front surface of the flexible sheet 50. Therein exists a problem such that, since the flexible sheet is flexible, distortion occurs before and after a pattern formation of a conductive film, and this is not suitable for a multilayer wiring structure since displacement between laminated layers is great.
In order to improve supporting strength to suppress the sheet distortion, it is necessary to sufficiently thicken the flexible sheet 50 to approximately 200 μm, and this goes against a reduction in thickness.
Furthermore, in terms of a manufacturing method, in the aforementioned manufacturing devices, for example, in the die bonder, wire bonder, transfer molding device, reflow furnace, etc., the flexible sheet 50 is transferred and attached to a part called a stage or a table.
However, when the thickness of an insulating resin to serve as a base of the flexible sheet 50 is reduced to approximately 50 μm, if the thickness of the copper foil pattern 51 formed on the front surface is also thin such as 9-35 μm, transferring characteristics are considerably inferior due to warping as shown in FIG. 18, and attaching characteristics to the aforementioned stage, or table are inferior, therein exists a drawback. This is considered to be warping owing to that the insulating resin itself is considerably thin and warping owing to a difference in the thermal expansion coefficient between the copper foil pattern 51 and insulating resin.
In addition, since the part of the opening portions 52 is pressured from the upside during molding, a force to warp the circumferences of the bonding pads 51A upward may act to deteriorate the bonding pads 51A in adhesive properties.
In addition, if the resin material itself to form a flexible sheet 50 lacks flexibility or if a filler is mixed to enhance thermal conductivity, the flexible sheet 50 becomes rigid. In this condition, when bonding is performed by a wire bonder, the bonding part may crack. In addition, during transfer molding, the part where the metal mold is brought into contact may crack. This appears more prominently if warping exists as shown in FIG. 18.
Although the flexible sheet 50 described above is a flexible sheet on whose rear surface no electrode is formed, an electrode 51D may be formed, as shown in FIG. 17C, on the rear surface of the flexible sheet 50, as well. In this case, since the electrode 51D is brought into contact with the manufacturing devices or is brought into contact with the transferring surfaces of the transferring means between the manufacturing devices, there exists a problem such that damage occurs to the rear surface of the electrode 51D. Since the electrode is formed with this damage included, there also exist problems, such that the electrode 51D itself cracks afterward by a heat application and solder wettability declines in a solder connection to a motherboard.
In addition, during transfer molding, a problem also occurs such that a sufficient sealing structure cannot be realized because of weak adhesive properties between the flexible sheet 50, copper foil pattern 51 and the insulating resin.